Typically in most fractionation processes, e.g. atmospheric fractionation of crude oil, it is very difficult and just about impossible to remove all of the water in the feed to the fractionation tower. In a typical tower, flashing of only one-third of a cubic foot of water in one second will cause a differential pressure of 1.5 to 2.5 pounds per square inch across each of the usual vertically stacked fractionation trays in the tower. Such a differential pressure is sufficient to distort and upset the trays and thus cause a loss of fractionation capacity in the tower. In such a process, water is typically found in the feed or can be trapped in the tower from washing operations or can result from condensing of stripping steam from which it cannot be completely eliminated and thereby avoid flashing and potential damage to the fractionation trays. In the event of damage to the tray as a result of a sudden pressure surge resulting from the flashing of the water, the fractionation capacity of the tower is impaired when the trays are damaged, a shutdown of the unit for tray repair or replacement is required. This repair causes loss in plant production and results in considerable mechanical maintenance costs.
While a large number of arrangements and variations for pressure surge relief have been attempted, they have been found to be uneconomical for use in existing fractionation towers since they cause loss in liquid-vapor contact area or fractionation capacity. Problems with the prior art have included high cost, insufficient vapor relief capacity, reduction of fraction capacity, coking in dead flow areas caused by the location of the pressure relief device and in general, prior art systems are not universally applicable to all types of fractionation towers. For example, the Chatfield U.S. Pat. No. 2,045,518 discloses a pressure relief arrangement for relieving pressure surges, but is only applicable to towers with bubble trays and not to present day towers in which all available tray space is used for vapor-liquid contact. Installation of this type of device in the present tower will severely reduce fractionation capacity and results in coking in the dead flow zones of the port covers on many surfaces. Further, these ports do not have pressure relieving capacity.
Parsons U.S. Pat. No. 2,105,501 discloses a circular pressure relief disc which blocks off a considerable portion of the fraction area of the tower and therefore is very uneconomical. Replacement in existing towers would require complete overhaul of tower internals, reduction in tower capacity and dead flow zones and coke formation in many areas.
Constantikes U.S. Pat. No. 2,809,821 has no pressure surge relieving capability, is extremely complicated and is an expensive tray to build.
Gilmore, U.S. Pat. No. 2,846,204 employs plate valves for providing vapor-liquid contacting and which also have the capability of absorbing some pressure surge; however, this prior art tray is extremely expensive and is not generally applicable to any service because of its special design. Typically, the valves would be unreliable and subject to sticking and jamming in a number of instances.
The Doering U.S. Pat. No. 3,105,862 employs variable opening jet tabs which possibly could absorb relatively small surges. However, such a tray employs only about 20% of its area for vapor liquid contact and cannot possibly absorb the magnitude of pressure surge absorbed by the present invention. The tray of this type is more expensive and extremely limited in utility, as well as subject to coking.
Other patents which have been located are Uitti U.S. Pat. No. 3,632,315 and Boon U.S. Pat. No. 3,649,466, neither of which disclose the structure or the pressure surge relieving capabilities of the present invention. Also, Hirao U.S. Pat. No. 3,807,711 and Tjutjunnikov et al. U.S. Pat. No. 3,817,501 disclose specific fractionation tray designs which attempt to improve vapor-liquid contacting over broad but limited ranges of vapor-liquid loadings. While it is possible that these disclosures could provide some slight pressure relieving capability, no disclosure is made thereof and they are not deemed to disclose the present invention. None of these patents disclose an arrangement or tray construction wherein the tray is returned to its normal operating position if a pressure surge occurs or restrains relatively minor undesirable variations in the vapor-liquid contact area during normal operation.